Chapter Seven: Energy ¾7.1 Energy and Systems ¾7.2 Conservation of Energy ¾7.3 Energy Transformations ENERGY Chapter 7.1 Learning Goals ¾Define energy as a description of an object’s ability to change or cause change. Investigation 7A Energy in a System ¾Key Question: How is energy related to motion? ¾Discuss examples of different forms of energy. ¾Distinguish potential and kinetic energy and apply formulas to solve problems. 7.1 What is energy? ¾ Energy measures the ability for things to change themselves or to cause change in other things. ¾ Some examples are changes in temperature, speed, position, pressure, or any other physical variable. 7.1 Units of energy ¾ Pushing a 1-kilogram object with a force of one newton for a distance of one meter uses one joule of energy. ¾ A joule (J) is the S.I. unit of measurement for energy. 1 7.1 Joules ¾ One joule is a pretty small amount of energy. ¾ An ordinary 100 watt electric light bulb uses 100 joules of energy every second! 7.1 Some forms of energy ¾ Chemical energy is a form of energy stored in molecules. ¾ Batteries are storage devices for chemical energy. 7.1 More forms of energy ¾ Nuclear energy is a form of energy stored in the nuclei of atoms. ¾ In the Sun, nuclear energy is transformed to heat that eventually escapes the sun as radiant energy. 7.1 Some forms of energy ¾ Mechanical energy is the energy possessed by an object due to its motion or its position. ¾ Potential energy and kinetic energy are both forms of mechanical energy. 7.1 Some forms of energy ¾ Electrical energy comes from electric charge, which is one of the fundamental properties of all matter. 7.1 More forms of energy ¾ Radiant energy is energy that is carried by electromagnetic waves. ¾ Light is one form of radiant energy. 2 7.1 More forms of energy ¾ The electromagnetic spectrum includes visible light infrared radiation (heat), and ultraviolet light. ¾ Light energy and heat energy are included in the electromagnetic spectrum. 7.1 Sources of energy ¾ Without the Sun’s energy, Earth would be a cold icy place with a temperature of -273 °C. 7.1 Sources of energy ¾ All objects with mass feel forces in the presence of Earth’s gravity. ¾ These forces are a source of energy for objects or moving matter such as falling rocks and falling water. ¾ As well as warming the planet, the Sun’s energy drives the entire food chain. 7.1 Energy and work ¾ In physics, the word work has a very specific meaning. ¾ Work is the transfer of energy that results from applying a force over a distance. 3 7.1 Potential energy ¾ Systems or objects with potential energy are able to exert forces (exchange energy) as they change. ¾ Potential energy is energy due to position. 7.1 Potential Energy mass of object (g) height object raised (m) PE (joules) EP = mgh gravity (9.8 m/sec2) 7.1 Kinetic energy 7.1 Kinetic Energy ¾ Energy of motion is called kinetic energy. ¾ A moving cart has kinetic energy because it can hit another object (like clay) and cause change. KE (joules) mass of object (kg) EK = ½ mv2 velocity (m/sec) Solving Problems ¾ A 2 kg rock is at the edge of a cliff 20 meters above a lake. ¾ It becomes loose and falls toward the water below. ¾ Calculate its potential and kinetic energy when it is at the top and when it is halfway down. ¾ Its speed is 14 m/s at the halfway point. 4 Solving Problems 1. Looking for: ¾ …initial EK, EP and EK, EP half way down. 2. Given: ¾ mass = 2.0 kg; h = 20 m ¾ v = 14 m/s (half way) 3. Relationships: ¾ EP =mgh ¾ EK = ½ mv2 ¾ Assume rock starts from rest. Chapter Seven: Energy ¾7.1 Energy and Systems ¾7.2 Conservation of Energy ¾7.3 Energy Transformations Solving Problems 4. Solution m = 20 kg EP = mgh ¾ Draw a free body diagram. EP = (2 kg)(9.8 N/kg)(20 m) h = 20 m EK = 0 J = 392 J at top EP = (2 kg)(9.8 N/kg)(10 m) = 196 J half way h = 10 m EK = 0 J, rock is at rest EP = mgh EK = ½ mv2 EK = (1/2)(2 kg)(14 m/s)2 = 196 J half way Chapter 7.2 Learning Goals ¾Describe how energy changes as systems change. ¾Discuss examples of energy transformations. ¾Explore the energy involved in carrying out daily activities. 7.2 Conservation of Energy ¾ Systems change as energy flows and changes from one part of the system to another. ¾ Each change transfers energy or transforms energy from one form to another. 5 7.2 Energy flow 7.2 Flow of Energy ¾ How can we predict how energy will flow? ¾ One thing we can always be sure of is that systems tend to move from higher to lower energy. 7.2 Sources of energy 7.2 Units of energy ¾ The chemical potential energy stored in the food you eat is converted into simple sugars that are burned as your muscles work against gravity as you climb the hill. ¾ Some units of energy that are more appropriate for everyday use are the kilowatt hour (kWh), food Calorie, and British thermal unit. Chapter Seven: Energy ¾7.1 Energy and Systems ¾7.2 Conservation of Energy ¾7.3 Energy Transformations 6 Chapter 7.3 Learning Goals ¾Explain what it means when energy is conserved. Investigation 7B Conservation of Energy ¾Key Question: What limits how much a system may change? ¾Use energy conservation to solve problems. ¾Discuss applications of energy conservation in daily living. 7.3 Conservation of Energy ¾ The idea that energy tranforms from one form into another without a change in the total amount is called the law of conservation of energy. ¾ The law of energy conservation says the total energy before the change equals the total energy after it. 7.3 Conservation of Energy ¾ When you throw a ball in the air, the energy transforms from kinetic to potential and then back to kinetic. Solving Problems ¾A 2 kg car moving with a speed of 2 m/sec starts up a hill. ¾How high does the car roll before it stops? 7 Solving Problems Solving Problems 1. Looking for: ¾ …height of hill 2. Given ¾ … mass = 2 kg, v = 2 m/s 3. Relationships: ¾ Energy transformed from EK to EP ¾ EK = ½ mv2 1. Solution ¾ Find beginning EK ¾ EK = ½ (2 kg) (2 m/s)2 = 4 Joules ¾ Assume energy before = energy after ¾ EK = EP ¾ EP =mgh 4 J = mgh ¾ h = (4 N•m)/(2 kg)(9.8 N/kg) = .2 m ¾ EP =mgh 7.3 Conservation of Energy ¾ Many people are concerned about “running out” of energy. ¾ What they worry about is running out of certain forms of energy that are easy to use, such as fossil fuels like oil and gas. 7.3 Conservation of Energy ¾ It took millions of years to accumulate these fuels because they are derived from decaying, ancient plants that obtained their energy from the Sun when they were alive. ¾ Because it took a long time for these plants to grow, decay, and become oil and gas, fossil fuels are a limited resource. 7.3 Conservation of Energy ¾ Regular (incandescent) light bulbs convert only 10% of electrical energy to light. ¾ That means 90% of the energy is released as wasted heat. 7.3 Conservation of Energy ¾ Other forms of energy, such as thermal energy, flowing water, wind, and solar energy are not as limited. 8 Investigation 7C Energy and Efficiency ¾Key Question: How well is energy changed from one form to another? A Matter of Survival ¾In 2005, the U.S. Defense Advanced Research Projects Agency (DARPA) launched its VHESC. program. ¾The goal of the program is to develop solar cells that would operate at or above 50 percent efficiency. 9